The invention relates to a hydraulic power transmission joint which is used to distribute a driving force of a vehicle and, more particularly, to a hydraulic power transmission joint which can freely control the transmission of torque to create a difference between rotational speeds of two power rotary shafts.
The present inventors have already disclosed in U.S. Pat. No. 5,037,353 such a transmission joint in which a plurality of plungers are arranged in the axial direction of the joint and in U.S. Pat. No. 5,103,642 such a transmission joint using a rotary valve.
In such a power transmission joint, an orifice is arranged in a path along which liquid is discharged by a plunger of a pump to an intake side of the pump, a high pressure is generated on the discharge side of the pump by the resistance offered by the orifice to the flow of the fluid, and the movement of the plunger is suppressed by the high pressure. Under such a condition torque is transmitted between two rotary shafts according to a rotational speed difference between the two rotary shafts.
In the above apparatus, the degree of opening of the orifice is fixed. However, if the degree to which the orifice was open could be changed, even in cases of the same rotational speed difference, the characteristics of the torque to be transmitted could be freely controlled.
When a vehicle is running off-road, it is desirable that the orifice be completely closed such that the joint assumes a locked state in which the motive power is transmitted to both the rear and front wheels whereby the vehicle is in a 4WD mode.
When the vehicle runs on a dry paved road, in order to reduce fuel consumption, it is desirable that the pump be rendered inoperative such that the joint assumes a free state in which torque is not transmitted, whereby the vehicle is in a 2WD mode.
For this purpose, a lock valve which can open and close the orifice or a free valve for rendering the pump temporarily inoperative are necessary.
In this case, it is necessary to provide an electromagnetic actuator or the like in order to make the degree to which the orifice is open variable or to actuate a free valve from outside of the joint.
The joint itself is coupled to two power transmission shafts. On the other hand, the electromagnetic actuator is connected to a controller by a signal line and is driven by an electric signal, so that the electromagnetic actuator needs to be fixed relative to the power transmission joint. Because a device for varying the effective opening of the orifice or a free valve of the rotating joint is driven by the fixed electromagnetic actuator, the structure of the overall apparatus is large.
When adjusting the degree of opening of the orifice with the electromagnetic actuator, it is necessary to switch a valve between at least open and closed positions. Further, switching at three or more levels is also required. It is generally sufficient to provide a plurality of electromagnetic actuators in order to effect such switching at the various levels. However, when such actuators are used, the apparatus is large and too complicated.
Since the valves provided in the joint are far from the electromagnetic actuator, in order to generate a sufficient electromagnetic driving force, a large solenoid coil is needed. Thus, the apparatus is large and consumes a large amount of electric power.
Further, it is difficult to provide the apparatus with a check for whether a valve provided in the joint has actually operated in accordance with a control signal. Reliability is thus poor. Although it is desirable to detect for a deviation of the valve from its normal operation, since parts of the joint are rotating and the valve mechanism is located in the joint, it is difficult to provide mechanical or optical devices for carrying out such detection. On the other hand, even if such a detecting capability is provided, it gives rise to increased costs of the apparatus.